The present invention relates to a pressure-relief valve device particularly, but not solely, for use with fluid-operated apparatus such as support props in mine workings.
Known forms of pressure-relief valve devices for use in mining employ a closure element such as a ball engaging on a valve seating under the action of a spring which maintains the valve closed unless a pre-determined pressure acts on the inlet of the valve to displace the closure element against the closure force. In one type of valve device described in German patent specification 1216057 the valve closure element is a shaped component with a conical sealing surface engaging on a seating formed at one end of a hollow sleeve in which a part of the closure element is guided.
The closing force is then applied to one end of this closure element by means of a piston-like thrust piece mounted in a bore and subjected to the force of the spring.
In order to accurately locate and support the components and to provide for a centralized application of the closure force to the closure element the necessary contact between the thrust piece and the closure element is essentially punctiform and this is achieved by having one of the mating surfaces convex. As a result of this arrangement the closure element is subjected to very high forces over a small area and this can lead to wear even over a short period of operation. Furthermore when the valve is opened and the guidance of the closure element in the sleeve is not so effective deformation can occur. A further disadvantage of this, and other known types of valve device, is that the valve surfaces, i.e. of the closure element and its seating, are usually subjected to wear due to particles and foreign matter impurities present in the hydraulic pressure fluid. Indeed in most cases the valves only open when some unusual condition produces a rise in the pressure of the fluid and hence over the long periods when the valves are closed a relatively large accumulation of dirt can occur which could affect the operation of the valve as well as produce high wear.
A general object of this invention is to provide an improved form of valve device.